|  | 1.6 | The Transport Layer of the OSI Reference Model
| 1.6.3 | The three-way handshake
To establish a connection, one machine places a call that must be accepted by the other. Protocol software modules in the two operating systems communicate by sending messages across the network to verify that the transfer is authorized and that both sides are ready. After all synchronization has occurred, a connection is established, and the transfer of data begins. During transfer, the two machines continue to communicate with their protocol software to verify that data is received correctly. The figure depicts a typical connection between sending and receiving systems. When you first meet someone, you often greet the person by shaking his or her hand. The act of shaking hands is understood by both parties as a signal for a friendly greeting. We speak of connections on the network in the same way. The first handshake, or greeting, requests synchronization. The second and third handshakes acknowledge the initial synchronization request, as well as synchronize connection parameters in the opposite direction. The final handshake segment is an acknowledgment used to inform the destination that both sides agree that a connection has been established. After the connection has been established, data transfer begins.